In field applications, chemical analysis instruments may be confronted with various complex mixtures regardless of indoor or outdoor environments. Such mixtures may cause instrument contamination and confusion due to the presence of molecular interferents producing signatures that are either identical to that of the chemical compounds of interest or unresolved by the analytical instrument due to its limited resolution. An interferent can also manifest its presence by affecting the limit of detection of the compound of interest. A multi-stage analysis approach may be used to reduce the chemical noise and produce enough separation for deterministic detection and identification. The multi-stage analysis may include either a single separation technique such as mass spectrometry (MS) in MSn instruments or a combination of different separation techniques.
Ion mobility spectrometry (IMS) utilizes relative low electric fields to propel ions through a drift gas chamber and separate these ions according to their drift velocity. In IMS, the ion drift velocity is proportional to the field strength and thus an ion's mobility (K) is independent of the applied field. In the IMS both analyte and background molecules are typically ionized using radioactive alpha or beta emitters and the ions are injected into a drift tube with a constant low electric field (300 V/cm or less) where they are separated on the basis of their drift velocity and hence their mobility. The mobility is governed by the ion collisions with the drift gas molecules flowing in the opposite direction. The ion-molecule collision cross section depends on the size, the shape, the charge, and the mass of the ion relative to the mass of the drift gas molecule. The resulting chromatogram is compared to a library of known patterns to identify the substance collected. Since the collision cross section depends on more than one ion characteristic, peak identification is not unique. IMS systems measure a secondary and less specific property of the target molecule—the time it takes for the ionized molecule to drift through a tube filled with a viscous gas under an electric field—and the identity of the molecule is inferred from the intensity vs time spectrum.
Other mobility-based separation techniques include high-field asymmetric waveform ion mobility spectrometry (FAIMS) also known as Differential Mobility Spectrometry (DMS). FAIMS or DMS is a detection technology which can operate at atmospheric pressure to separate and detect ions. Compared to conventional ion mobility, FAIMS/DMS devices operate at much higher fields (˜10,000 V/cm) where ion mobilities become dependent on the applied field. FAIMS/DMS devices may operate in conjunction with IMS drift tube devices in spectrometers having multiple stages. For specific descriptions of features and uses of instruments for ion detection and chemical analysis, including features of IMS drift tube devices used in connection with one or more FAIMS/DMS devices, among other components, reference is made to U.S. Pat. No. 8,173,959 B1 to Boumsellek et al., entitled “Real-Time Trace Detection by High Field and Low Field Ion Mobility and Mass Spectrometry,” U.S. Pub. No. 2012/0273669 A1 to Ivashin et al., entitled “Chemical Analysis Using Hyphenated Low and High Field Ion Mobility,” and U.S. Pub. No. 2012/0326020 A1 to Ivashin et al., entitled “Ion Mobility Spectrometer Device with Embedded FAIMS,” which are all incorporated herein by reference.
Known atmospheric pressure ionization devices, such as the ones used in IMS and DMS devices, may use a radioactive ionization source to generate the ions that are used in connection with the chemical analysis and detection processes. Still other known devices may use non-radioactive ionization techniques that include corona discharges and/or ultraviolet (UV) light and laser-induced ionization. In connection with the above-noted techniques, reference is made, for example, to U.S. Pat. No. 8,440,981 to Bromberg et al., entitled “Compact Pyroelectric Sealed Electron Beam,” U.S. Pat. No. 6,429,426 to Döring, entitled “Ionization Chamber with Electron Source,” and U.S. Pat. No. 5,969,349 to Budovich et al., entitled “Ion Mobility Spectrometer,” all of which are incorporated herein by reference.
Fieldable trace detection of illicit substances, particularly explosives and narcotics, is challenging primarily due to the wide range of volatility and to the electro-chemical properties of these compounds. While common explosives consist of nitro compounds detectable in negative mode since they form stable negative ions under condition of ambient pressure, some emerging higher volatility improvised explosives devices (IEDs) and homemade explosives (HMEs) are known to have high proton affinities in the form of adduct ions sometimes in the presence of chemical modifiers.
Accordingly, it would be desirable to provide for ion analysis techniques using an ionization source that provides the flexibility required to optimize the detection performance for a broad range of substances with different physical and chemical properties.